1. Field of the Invention
The present invention relates generally to a silicone-coated polymeric film having a back side coating effective to reduce the undesirable slip generated by the by-product or contaminant silicone oil released from the silicone coating. It further relates to a method for controlling undesirable slip in silicone-coated polymeric film by coating the back side of the film with a slip control coating.
2. Description of Related Art
Silicone-coated polymer films are well known and commercially available. Such silicone-coated polymer films are commonly employed as release films for labels, pressure sensitive tapes, decorative laminates, transfer tapes and the like. The silicone coating on the base polymer film allows the adhesive face of the label, for example, to be easily removed from the support film by the end user, while providing sufficient stability to the label to prevent it from being accidentally dislodged from the support film before use. Among other things, silicone release coatings can also be used as container liners designed for contact with food. Pizza box liners coated with silicone release coatings, for example, allow melted cheese to release easily. Food waste is minimized, the food is served more easily, and the appearance of the product is maintained.
A variety of different silicone release coatings are known in the art. These silicone release coatings include crosslinked formulations, ultraviolet curable or heat curable formulations, solvent-based and solvent-free formulations, and combinations of the foregoing, such as solvent-free, ultraviolet curable formulations. As disclosed in U.S. Pat. No. 5,350,601 to Culbertson et al., incorporated herein by reference in its entirety, primer formulations have been discovered that allow these silicone release coatings to bond effectively with the base polymer films, and to remain bonded over time to the base polymer film. These primer formulations are typically applied to the base polymer film, and the silicone release coating is applied on top of the resulting primer layer. The resulting coated polymer film provides superior performance. For example, coating rub-off, which can cause labels to release too easily, is minimized. Moreover, coating delamination, which can cause labels to adhere directly to the underlying base polymer film, is also minimized or eliminated.
However, during manufacture and during storage of the coated film prior to use, free silicone oil is inevitably released from the coating layer onto the surface of the coated film. This migration creates a high slip layer or discrete patches of free silicone oil on the film surface. Free silicone oil can also be transferred easily to the non-silicone-coated back side of the film. When, for example, the coated film is stored in front-to-back-face contact, as in a typical roll of film, some of the free silicone oil (typically low molecular weight silicones) on the front, silicone coated surface of the film will be transferred to the back (typically uncoated) surface of the roll. In the film roll, which is typically formed during manufacture for storage or transport, the front, silicone coated surface is placed in contact with the back surface of the same web as it is wound.
In practice, this free silicone oil contamination can be a drawback. For example, when the coated film traverses over idler rolls during subsequent processing steps (e.g., during label application), the idler rolls can slip and lose contact with the film. This can result in uneven tension on the film web or loss of alignment. Related problems arise when printing the resulting silicone-coated film or labels applied to that film. The ability of printing inks and solvents to adhere is impaired by the presence of free silicone oil on the surface to be printed. Moreover, loss of alignment due to the film""s high slip can lead to a high reject rate during printing, particularly where multiple printing passes are used and proper alignment is crucial to forming a n integrated final image.
The prior art has recognized the existence of the free silicone oil on the surfaces of the coated film, and the resulting transfer contamination problem. However, no suggestion has been made in the known art of any approach to controlling the free silicone oil generated by the coated film. Instead, in an implicit acknowledgment that no solution to the transfer contamination problem was available or evident, prior art patents (e.g., U.S. Pat. No. 5,110,671 to Balloni et al. and U.S. Pat. No. 4,961,992 to Balloni et al.) have attempted to use the free silicone oil and the resulting increased slip to improve processing.
Nonetheless, it remains commercially desirable to control the effects of free silicone oil. Film processing parameters are highly sensitive to changes in film friction properties. Ideally, silicone-coated films would have a coefficient of friction (xe2x80x9cCOFxe2x80x9d) equivalent to that of plain film to simplify processing. This would eliminate the costly and time consuming equipment and setting modifications that are required to process film having substantially different COFs, as well as the higher reject rate inherent in processing under significantly different COF conditions.
U.S. Pat. No. 5,350,601 to Culbertson et al., discussed above, discloses a silicone-ready primer coating including a glycidoxy silane and a copolyester. This primer coating is designed for use under a silicone coating, to improve the integrity of the silicone coating on a base polymer film. While the Culbertson et al. patent makes reference to coating both sides of the film with the primer coating, it is envisioned only that a silicone coating will be applied to all primer coated surfaces. There is no suggestion that one side would be coated while the other would be left uncoated. More importantly, there is no suggestion in the Culbertson et al. patent of the silicone-binding properties of the silane and copolyester coating.
A commercial product sold at least as early as one year prior to the filing date of the present application by the assignee of the present invention under the designation Hostaphan(copyright) 3SAC polyester film included a silicone-ready primer coating on both sides. That silicone-ready primer coating included an isophthalic acid/sodium 5-sulfoisophthalic acid/malonic acid/ethylene glycol copolyester, a glycidoxypropyltrimethyoxysilane, a colloidal SiO2 and sodium lauryl sulfate. However, neither side of the film, as sold by the assignee, was silicone coated. The film was sold as a silicone-ready film, and purchasers of the film are known to have coated it with silicone. However, the inventors are not aware of any purchaser who coated one side with silicone, but not the other. Alternate films with the silicone-ready coating on a single side only (e.g., 3SAB polyester film) were available to customers not interested in silicone coating both sides.
Accordingly, it is an object of the present invention to provide a base polymer film with a coating capable of attracting and tying up free silicone oil contaminants.
It is a further object of the present invention to provide a base polymer film with a slip control coating on the opposite side of the base polymer film from the silicone coating. This slip control coating can minimize slippage caused by free silicone oil released from the silicone coating.
It is another object of the present invention to provide a back side coating that provides a resulting film having a COF equivalent to that of plain, uncoated polymer film.
It is yet another object of the present invention to provide a method of controlling slippage on silicone-coated film by means of a reverse-side coating.
The present invention has accomplished these objectives by providing a self-supporting polymer film with a silicone-containing coating layer on the front surface that is capable of releasing free silicone oil and a slip control coating on the back surface, where the slip control coating has an exposed surface adapted to contact the free silicone oil. The slip control coating includes a binder, preferably including a water-soluble copolyester including about 50 to about 98 mol percent isophthalic acid, about 2 to about 20 mol percent of at least one sulfomonomer containing a sulfonate group attached to a dicarboxylic nucleus, and about 100 mol percent of at least one copolymerizable glycol having from about 2 to about 11 carbon atoms; and also includes a coupler, preferably including a silane. An alternate preferred binder is polyvinyl pyrrolidone (PVP). A method for controlling the slip of silicone coated film by coating the back side of the film with this slip control coating is also disclosed.
Slip Control Coating
The slip control coating of the present invention acts as an absorbent that is able to tie up substantial amounts of free silicone oil. Thus, when free silicone oil is released from a silicone coating applied to the base polymer film, the slip control coating is able to absorb or otherwise take up, tie up, deactivate or render less lubricant some or all of that free silicone oil. Consequently, the COF of the resulting film is increased, allowing the film to be processed more easily and evenly.
Most preferably, the slip control coating is applied to a surface (for reference purposes, referred to herein as the back surface) of the base polymer film opposite the surface (the front surface) bearing the silicone coating. When the film is wound up into a roll for further processing, transport or storage, the front and back surfaces come into close and direct contact. The slip control coating can then accept the loose silicone released from the silicone coating, and prevent or minimize the formation of patches of free silicone oil on either the front or the back surface of the film. While it is preferred that the slip control coating be applied evenly over the entire back surface of the film, it is also envisioned that the absorbent slip control coating may be applied in selected areas or patterns, or at varying thicknesses, or a combination thereof.
The slip control coating of the present invention preferably includes a binder to anchor the slip control coating to the base polymer film. One preferred binder is a water soluble copolyester. Preferably, the water soluble copolyester includes a copolyester as disclosed in U.S. Pat. No. 4,493,872 to Funderburk et al., the disclosure of which is incorporated herein by reference in its entirety. This copolyester is disclosed as the condensation product of the following monomers or their polyester forming equivalents: isophthalic acid, a sulfomonomer containing an alkali metal sulfonate group attached to a dicarboxylic aromatic nucleus and an alkylene glycol with about 2 to about 11 carbon atoms. Optionally, an aliphatic dicarboxylic acid of the formula HOOC(CH2)xe2x80x94nCOOH, where n is about 1 to about 11, can also be employed as a monomer therein. An optimal copolyester is made up of about 90 mol percent isophthalic acid, about 10 mol percent of the sodium salt of 5-sulfoisophthalic acid and about 100 mol percent ethylene glycol. Alternatively, the ethylene glycol can be replaced with a propane diol, such as 1,3-propane diol, or combinations of the two can be used.
It is important to note, however, that the preferred percentage of sulfomonomer, isophthalic acid and aliphatic dicarboxylic acid employed is somewhat broader in the context of the present invention than in the Funderburk et al. patent. For example, in the context of the present invention, isophthalic acid is preferably about 40 to about 98 mol percent, aliphatic dicarboxylic acid is preferably about 0 to about 50 mol percent, and the sulfomonomer is preferably about 2 to about 20 mol percent. In addition, the sulfomonomer group of the present invention is not limited to an alkali metal sulfonate group. Any sulfomonomer in which a sulfonate group is attached to a dicarboxylic nucleus is preferred for use herein. In fact, any water soluble copolyester that functions to bind the coating to the surface of the base polymer film, either alone or synergistically in combination with other components, is preferred for use in the slip control coating of the present invention.
An alternate preferred binder component for use in the present invention is polyvinyl pyrrolidone (PVP). Preferably, a polyvinylpyrrolidone, PVP K-15(copyright), from ISP, is used. It has also been found that PVP (and less preferably, the copolyester binder) are preferably used in conjunction with a plasticizer, in part to enhance the coating coalescence. This plasticizer preferably includes glycerol, ethylene glycol, or a combination of the two. The binder may also include a water soluble copolyester and PVP.
The binder is preferably present at about 0.1 to about 25 weight percent of the slip control coating composition. It is more preferably present at about 1 to about 10 weight percent, and most preferably at about 2.6 weight percent. However, a PVP binder is most preferably present at about 10 weight percent. When a plasticizer is used in combination with the PVP, it is preferably present at about 0.01 to about 5 weight percent, more preferably about 0.1 to about 2 weight percent.
It is also preferred that the slip control coating of the present invention include a coupler component to couple with or otherwise absorb or tie up the free silicone oil contaminant. Preferably, the coupler includes a silane component. More preferably, the slip control coating includes a glycidoxy silane, most preferably a glycidoxypropyltrimethoxysilane, such as the one commercially available as Dow Corning Z-6040(copyright) glycidoxypropyltrimethoxysilane.
The coupler component is preferably present at about 0.1 to about 10 weight percent of the slip control coating composition, more preferably at about 0.5 to about 5 weight percent. Most preferably, the coupler component is present at about 1.4 weight percent.
The preferred proportion of the coupler to the binder, when the two are combined in the slip control coating of the present invention, is in the range of about 1:3 to about 3:1 by weight. Although the scope of the present invention is intended to cover even broader ranges, it has been found that these proportions provide better results. More preferably, the slip control coating will include about 1 part coupler to about 2 parts binder by weight.
In addition, a wicking aid is preferably incorporated into the slip control coating of the present invention. The wicking aid is believed to enhance the ability of the slip control coating to absorb or otherwise take up free silicone oil. The wicking aid is preferably inorganic. More preferably, the wicking aid includes colloidal SiO2, most preferably the product commercially available as Nalco 1060(copyright) colloidal SiO2. Other wicking aids that are preferred for use in the present invention include silica in one or more of its various morphological forms, including those commercially available as Syloid(copyright) silica or Rapidup(copyright) silica. Moreover, a combination of two or more of the foregoing wicking aids is also preferred for use. The wicking aid is preferably present at about 0.01 to about 3 weight percent of the slip control coating, more preferably at about 0.1 to about 1 weight percent. Most preferably, the wicking aid is present at about 0.4 weight percent.
Moreover, the slip control coating of the present invention preferably includes a surface active agent. It is believed that this surface active agent enhances the appearance of the slip control coating when it is coated onto the base polymer film. More preferably, the surfactant includes an anionic surfactant, most preferably sodium lauryl sulfate. Alternate preferred surfactants that have been shown effective in the compositions of the present invention include fluorosurfactants, including the surfactant commercially available from 3M as Fluorad(copyright) FC-170C; sodium dodecylbenzene sulfonate, including the product commercially available as Rhodacal(copyright) LDS-10 surfactant from Rhone Poulenc; ethoxylated 2, 4, 7, 9-tetramethyl-5-decyl-4, 7-diol, including the product commercially available from Air Products as Surfynol(copyright) 465. In addition to other beneficial properties, these surfactants have been shown to provide enhanced resistance to blocking. It is preferred that the surfactant be present in the slip control coating at about 0.0001 to about 5 weight percent, more preferably at about 0.001 to about 1 weight percent. Most preferably, the surfactant is present at about 0.04 weight percent.
It has also been found that the pH of the slip control coating affects its performance. Preferably, the pH of a copolyester-based and/or silane-based slip control coating, as applied, is about 7 to about 10, more preferably basic, from about 7.5 to about 9. Consequently, the selection of alkaline components, such as alkaline wicking aid Nalco 1060(copyright) colloidal SiO2, will affect the pH of the resulting slip control coating, and thus its performance. Conventional pH modifiers may preferably be used to the extent necessary to achieve the desired pH levels, alone or in combination with alkaline wicking aids. Other conventional additives may also be used in the slip control coating of the present invention.
The slip control coating of the present invention is typically applied to the base film as an aqueous dispersion and at a solids concentration in the range of about 0.5 to about 12 weight percent, more preferably at about 2 to about 6 weight percent. The preferred solids level of the slip control coating, as it is applied to the base polymer film, is a level sufficient to yield a final dry coating thickness within the range of about 0.03 g/m2 to about 0.15 g/m2. The preferred thickness of the dried slip control coating is about 2.5xc3x9710xe2x88x929 m to about 2.5xc3x9710xe2x88x926 m with a more preferred thickness of about 2.5xc3x9710xe2x88x928 m.
While the COF of the back surface of the film, measured against itself (back-to-back) is highly dependent on variable manufacturing and processing analyses, it can generally be said that a back-to-back COF of greater than about 0.30 is preferred for coated films of the present invention, with back-to-back COF of greater than about 0.35 being more preferred, and greater than about 0.40 being most preferred. Expressed in a relative fashion, it is preferred that the coated films of the present invention have a back-to-back COF at least about 0.10 greater than the same silicone-coated film without a slip control coating of the present invention.
Base Film
The slip control coatings and coatings method of the present invention are applicable to any polymeric film capable of acting as a substrate for a silicone-release coating and a slip control coating. For example, the present invention is applicable to polymeric films such as those made from polyamides exemplified by nylon; polyolefins such as polypropylene and polyethylene; polyester such as polyethylene terephthalate; polyacetal; polycarbonate; and the like. The invention is particularly applicable to polyester, most preferably polyethylene terephthalate, polyethylene naphthalate or polybutylene terephthalate. The present invention is also applicable to polymeric films including copolyesters such as polyethylene terephthalate isophthalate. A preferred process for forming a base film is set forth in U.S. Pat. No. 5,350,601 to Culbertson et al., incorporated above by reference. Generally, any polyester film based on a polymer resulting from polycondensation of a glycol or diol with a dicarboxylic acid (or its ester equivalents) such as terephthalic acid, isophthalic acid, sebacic acid, malonic, adipic, azelaic, glutaric, suberic, succinic acids and the like, of mixtures of two or more of the foregoing, are preferred for use in the present invention. Suitable glycols include ethylene glycol, diethylene glycol, polyethylene glycol, and polyols such as butanediol and the like. Mixtures of two or more of the foregoing are also suitable.
Any of the above base polymer films can contain conventional additives such as antioxidants, delusterants, pigments, fillers such as silica, calcium carbonate, kaolin, titanium dioxide or mixtures thereof, antistatic agents and the like, all of which are well known in the art.
In addition, the base polymer film may be a polymer laminate. Such laminates include polymer-polymer laminates like polyester-polyolefin or polyester-adhesive-polyolefin, polymer-metallic laminates such as polyester-aluminum, or polymer-paper or polymer-adhesive-paper laminates.
The films may be produced by any well known technique in the art. For example, polyester is typically melted and extruded as an amorphous sheet onto a polished revolving casting drum to form a cast sheet of the polymer. The sheet is quickly cooled and then stretch oriented in one or more directions to impart strength and toughness to the film. The sheet is typically stretched from about two to about four times the original cast sheet dimension, in one or both directions. Biaxial orientation is most preferred, with monoaxial orientation being less preferred. Generally, stretching occurs in a temperature range from about the second order transition temperature of the polymer to below the temperature at which the polymer softens and melts. Where necessary, the film is heat treated after stretching to xe2x80x9clock-inxe2x80x9d the properties by further crystallizing the film. The crystallization imparts stability and good tensile properties to the film. Such heat treatment for polyester film is generally conducted at about 190xc2x0 C. to about 240xc2x0 C.
Silicone Coating
The silicone coatings preferred for use in the present invention are silicone release coating compositions. A variety of silicone release coating compositions are commercially available, with preferred compositions including those sold by Wacker Silicones Company, General Electric Silicones, PLC (Rhodia), Dow-Corning and others. The silicone coating can be, but is not limited to, a solvent cross-linkable type silicone coating, a solvent-free silicone coating, a solvent-free ultraviolet or electron beam curable silicone coating, or an aqueous based silicone coating. The slip control coating of the present invention is useful in conjunction with any silicone coating that releases free silicone oil. Thus, any films coated with silicone coatings that generate free silicone oil contaminants can also benefit from use with the back side coatings and coating methods of the present invention.
A preferred silicone coating for use in the present invention is a thermal cured silicone coating, more preferably a platinum catalyzed coating. Most preferably, the silicone coating used is the silicone coating present on the polyester film commercially available from Mitsubishi Polyester Film, LLC as Hostaphan(copyright) 2SLK. It is preferred that these be contained in a silicone emulsion. It is, of course, important that the slip control coating not interfere substantially with the curing and anchoring ability of the silicone coating on the front side. Accordingly, the compatibility of the slip control coating with the specific silicone coating must be considered. It is preferred that the silicone coating be fully cured.
Coating Methods
In-line coating of the base polymer layer, in which the coatings are applied during the film manufacturing process and before it is heat-set, is the preferred method for use herein. Typically, the base polymer film is coated after corona treatment and prior to the stretch orientation of the film as described in British Patent No. 1,411,564, or coated between drawing steps (when biaxially oriented film is produced) as taught by U.S. Pat. No. 4,571,363, or coated post-draw as taught by U.S. Pat. No. 3,322,553.
In addition to in-line coating, one or more of the coatings of the present invention may be off-line coated (after manufacturing and heat setting the film), preferably after conventional surface modification of the polymeric substrate has occurred. Thus, the coating and method of the present invention are also intended for use where, for example, the base polymer film is produced and later coated off-line with one or more coatings of the present invention. Alternatively, one or more coatings can be applied in-line, with the remainder being applied off-line. Conventional off-line coating processes include roll coating, reverse roll coating, gravure roll coating, reverse gravure roll coating, brush coating, wire-wound rod (Meyer rod) coating, spray coating, air knife coating, meniscus coating or dipping.
While surface modification of the base polymer film prior to coating is not required, it has been found that better results are obtained if the surface or surfaces of the base polymer film are modified before application of the coatings of the present invention. Conventional surface modification techniques include corona treatment, which is the most common and most preferred procedure for modifying the surface of the polymer base film to enhance coating adhesion. The corona treatment or other surface modification should be sufficient to permit wetting out of the coating. Corona treatment of about 1.0 watts per square foot per minute is typically sufficient to achieve the desired results.
In light of the foregoing, a preferred method of controlling slip of a silicone-coated film is provided herein. Preferably, a back face of a base polymer film is coated with a slip control coating of the present invention. This coating can occur before, after or at the same time the front face of the base polymer film is coated with the silicone coating. The slip control coating is applied at a stage before the silicone coating is applied, or it can be applied after the silicone coating is applied. The slip control coating is preferably not overcoated with another coating. Such a top coating could limit the ability of the slip control coating to tie up or otherwise limit the slip of the free silicone oil contaminants. Moreover, it is possible to coat layers of individual components or submixtures of the slip control coating in sequence on a base polymer layer to achieve the objectives of the coating. Such layering would be encompassed within the scope of the present invention.